1. Field of the Invention
The present invention relates to an image formation apparatus, using a contact charging method, which is capable of preventing the generation of ozone and NOx components in the course of corona charging, using a non-ozone-generating gas with the prevention of the dissipation thereof from a corona a charging area with a simple mechanism.
2. Discussion of Background
In image formation apparatus such as copying machines, printers and facsimile apparatus, images are formed by electrophotography, which comprises a series of processes of uniformly charging a surface of a photoconductor whose surface resistivity changes depending upon the intensity of light applied thereto, forming a latent electrostatic image corresponding to an output image on the uniformly charged surface of the photoconductor, with the application of a laser beam thereto, developing the latent electrostatic image to a visible toner image with a toner which is electrically charged, transferring the developed toner image to a charged transfer sheet such as a sheet of paper, quenching the charges of the transfer sheet, peeling the transfer sheet away from the surface of the photoconductor, and fixing the toner image on the transfer sheet thereto with the application of heat and pressure thereto. After the above-mentioned series of processes in electrophotography, a residual toner remaining on the surface of the photoconductor is removed, residual charges on the surface of the phoconductor are quenched, and then the surface of the photoconductor is uniformly charged for the next image formation.
In electrophotography, the movement of charges is utilized in each of the processes of charging, development, image transfer, and charge quenching, and the generation of charges is carried out, for example, by a corona charging method, a triboelectric charging method, or a contact charge injection method. Of these charge generation methods, the corona charging method is most in general use.
In the corona charging method, corona charges are generated with the application of a high voltage across an electrode made of a thin wire or a stylus and a counterelectrode, and ions generated by the corona charges are applied to a chargeable member such as a photoconductor. The principle of the corona charging method is very simple and the structure of an apparatus to perform the corona charging method is also very simple. However, since the corona charging is carried out in air, oxygen which occupies 20% of the components of air is ionized, so that ozone (O.sub.3) is generated. Ozone is an important compound for the ozone layer in the stratosphere which acts as a shield against penetration of UV light in the sun's rays. However, ozone is toxic in offices and the generation thereof must be controlled.
The triboelectric charging method and the contact charge injection method are applied to a development roller and a charging roller. However, in the triboelectric charging method and the contact charge injection method, since the development roller or the charging roller remains in contact with the surface of a photoconductor which is a chargeable material even when the method is not carried out, low-molecular-weight components separate out from a rubber roller of the roller and are transferred to the photoconductor, whereby the photoconductor is contaminated with such low-molecular-weight components, eventually causing abnormality in image formation. The triboelectric charging method and the contact charge injection method have such a shortcoming as mentioned above.
Under such circumstances, recently a contact charging method is employed, in which the surface of a charging member is caused to have high resistivity, and a charging portion of such a charging member is successively brought into contact with the surface of a chargeable member such as a photoconductor, so that corona charging is conducted in a micro space around the charging portion of the charging member which is in contact with the chargeable member, whereby the surface of the chargeable member is uniformly charged with the application of charges thereto.
Even in the above-mentioned contact charging method, however, as long as corona charging is used, oxygen in air is ionized, so that ozone and NOx components are inevitably generated. The NOx components are hygroscopic, so that when the NOx components are deposited on the surface of the photoconductor, abnormal images with image flow are formed. Furthermore, when the NOx components are deposited on the surface of the photoconductor or the surface of a charging roller, and the charging roller comes into contact with the photoconductor, it may occur that low-molecular components are transferred from the charging roller to the photoconductor. When such transfer of the low-molecular components occurs and a copying operation is resumed, non-transferred spots are formed in copied images.
Therefore it is desired that a gas that hinders the generation of ozone in the charging atmosphere be developed. As one of the proposals for attaining this, a method of using an oxygen-concentration-reduced air is proposed in Japanese Laid-Open Patent Application 60-95459. When the concentration of oxygen in air is merely reduced, the amount of ozone generated in the course of corona charging can be reduced. However, the generation of ozone cannot be stopped completely. Furthermore, the electric current in the corona charging varies depending upon the kind of gas employed. The result is that charging potential varies depending upon the kind of gas employed and accordingly image density varies. In order to prevent such problems, it is necessary to use a gas-separation filter, which will make the charging apparatus complicated in mechanism.
Therefore it is desired to produce a charging atmosphere free of oxygen or a charging atmosphere in which ozone is not generated by corona charging even if oxygen is contained therein.
It is considered that nitrogen gas (N.sub.2) which is a main component of air and is easily available can be used for producing the above-mentioned charging atmosphere. However, nitrogen gas (N.sub.2) has a density which is close to the density of oxygen gas, so that nitrogen gas (N.sub.2) easily disperses in air. Therefore, in order to produce and maintain a charging atmosphere composed of pure nitrogen gas, a special apparatus or a nitrogen gas supply apparatus is required.
Furthermore, as described in Japanese Laid-Open Patent Application 60-95459, NOx components are produced by corona charging in an atmosphere of nitrogen, so that the above-mentioned problems such as the increase of the hygroscopic properties of the photoconductor, and the reduction of the charging performance of the photoconductor are caused. Therefore such a charging method in which NOx components are produced should not be used.
As easily available non-ozone-generating gases, there are water vapor H.sub.2 O, hydrogen gas H.sub.2, rare gases such as He, Ne, propane gas C.sub.3 H.sub.8, and methane gas CH.sub.4. As a matter of course, gases which catch fire cannot be used, and materials which are not in the state of a gas at room temperature cannot be used, either.
When a non-ozone-generating gas which is lighter than air is employed, a container for the non-ozone-generating gas by which the dissipation of the gas from the corona charging area can be prevented, or some device for continuously supplying the gas to the corona charging area is required.